1. Field of the Invention
The invention relates to the field of transformers, particularly transformers useful in connecting a twisted pair transmission line to a transceiver.
2. Prior Art
In many applications where transformers are used for isolation, there is a need to reduce common mode noise coupling between windings and at the same time minimize so-called xe2x80x9copen framexe2x80x9d noise caused by magnetic flux from other devices inducing voltages in the windings of the transformer. This is particularly true where transformers are used to isolate a twisted pair communication line, such as a telephone line, from a transceiver.
One of the most effective ways to reduce noise pick-up from stray magnetic flux in transformers is to use a toroidal core with windings uniformly disposed around the full circumference of the toroid. Multiple windings are either wound on top of each other in layers or wound at the same time in a bifilar fashion. Uniformly spreading each winding around the full circumference of the toroid results in cancellation of stray magnetic field pick-up. This is true since windings on opposite sides of the toroid induce opposite polarity signals. Another advantage to tightly coupling primary and secondary windings is that leakage inductance is significantly reduced. Low leakage inductance provides wider transformer bandwidth.
Common mode noise coupling is generally the result of the parasitic capacitive coupling between the windings of the transformer. This capacitance can be most easily reduced by separating the windings, such as by having one winding disposed on one sector of the same core and a second winding disposed on another sector of a toroidal core. With this sector winding technique, the distance between windings is increased and consequently the capacitance between the windings is reduced. In some applications a transformer may need to provide a safety isolation barrier between windings of a transformer in addition to functional isolation requirements. Meeting stringent safety agency insulation and voltage breakdown requirements also leads to greater separation between transformer windings.
Thus, when windings are sector wound, common mode noise coupling is reduced and safety isolation requirements facilitated but noise pick-up from stray magnetic fields is increased and transformer bandwidth is reduced. Conversely, steps taken to reduce noise pick-up from stray magnetic fields tend to increase common mode noise coupling.
As will be seen, the present invention provides a transformer with a separator between the primary and secondary windings which reduces the capacitance and hence the common mode noise and at the same time minimizes noise pick-up from stray magnetic fields and transformer leakage inductance. This same construction also facilitates stringent safety isolation requirements without compromising other functional parameters.
A transformer is described having a toroidal core and a first winding disposed about the core. A winding separator comprising a pair of annular-shaped cups encircles the first winding. A second winding is disposed about the outer surface of the separator.
In one embodiment a tab extends adjacent an opening which provides access to the interior of the winding separator. The ends of the first winding pass through the opening and are separated from the second winding by the tab. In another embodiment, the interior of the separator includes spacers to provide a further air gap between the first and second windings.